U.S. patent number 3,889,067 [Application Number 05/409,062] was granted by the patent office on 1975-06-10 for digital telephone switching system.
This patent grant is currently assigned to Technology Service Corporation. Invention is credited to Irving R. Obenchain, Jr., Irving S. Reed.
United States Patent |
3,889,067 |
Reed , et al. |
June 10, 1975 |
Digital telephone switching system
Abstract
A digital telephone switching system is disclosed in which voice
or other communication signals are converted to digital signals and
switched in the digital form. The digital signals are reconverted
to analog signals and sent out on the selected line or trunk. To
accomplish that, dialing signals (pulses or tones) are used to
enable a selected AND gate in a matrix for transmission from the
calling line to a line called, using an analog-to-digital converter
on the calling line and a digital-to-analog converter on the called
line. An AND gate selectively enabled in the same manner in a
second matrix provides transmission from the called line to the
calling line, again using analog-to-digital and digital-to-analog
converters. For two-wire circuits hybrid junctions couple the
calling line and the called line to the analog ends of associated
analog-to-digital and digital-to-analog converters. In a second
embodiment, the digital switching matrices are implemented by the
use of time division multiplexing between input and output
registers. In either embodiment the switching action could be
controlled by a programmable digital computer.
Inventors: |
Reed; Irving S. (Santa Monica,
CA), Obenchain, Jr.; Irving R. (Pacific Palisades, CA) |
Assignee: |
Technology Service Corporation
(Santa Monica, CA)
|
Family
ID: |
23618900 |
Appl.
No.: |
05/409,062 |
Filed: |
October 24, 1973 |
Current U.S.
Class: |
379/250; 379/271;
379/284; 370/384 |
Current CPC
Class: |
H04Q
11/04 (20130101) |
Current International
Class: |
H04Q
11/04 (20060101); H04m 003/00 () |
Field of
Search: |
;179/18J,15AT,1AP |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Blakeslee; Ralph D.
Attorney, Agent or Firm: Lindenberg, Freilich, Wasserman,
Rosen and Fernandez
Claims
What is claimed is:
1. In a telephone system of the type in which dialing or other
control signals are used to determine the connection to be made
between any one of a plurality of telephone lines calling another
line selected from the same or other plurality of telephone lines,
a switching matrix at one site interfacing with standard two or
four-wire telephone transmission lines for switching from one line
to another communication signal of the form produced by the user
independently of the switching matrix, said switching matrix
comprising
means responsive to a calling line initiating a call for generating
a digital code, X.sub.M, uniquely identifying said calling
line,
means responsive to control signals received from said calling line
for generating a digital code, X.sub.N, uniquely identifying
another line called,
a plurality of converting means, one for each of said plurality of
lines which may be calling, each converting means serving to
convert communication signals received from its associated line to
digital form,
a plurality of reconverting means, one for each of said plurality
of lines which may be called, each reconverting means serving to
convert communication signals received to analog form for
transmission through its associated line, and
switching means resposive to said digital codes X.sub.M and X.sub.N
for routing communication signals in digital form from a converting
means associated with a calling line to a reconverting means
associated with a called line, all of said converting, switching
and reconverting means being at said one site, whereby all
transmission to and from said switching matrix is in digital
form.
2. The combination defined by claim 1 including a second similar
switching matrix at said one site and responsive to said digital
codes for switching communication signals from said called line to
said calling line.
3. The combination of claim 2 wherein each of said switching
matrices comprises
a plurality of incoming bus bars, one for each incoming line, and a
plurality of outgoing bus bars, one for each outgoing line, where
an incoming line is a calling line in one switching matrix and a
called line in the other, and an outgoing line is a called line in
said one switching matrix and a calling line in the other,
separate gating means for separately connecting each incoming bus
bar to each outgoing bus bar such that for any outgoing bus bar
selected at random, each one of the incoming bus bars is connected
to it by a separate gating means, each gating means being
responsive to a switching control signal to enable it to translate
digital signals from an incoming bus bar to an outgoing bus bar,
and
means responsive to said digital codes X.sub.M and X.sub.N for
generating a control signal S.sub.MN at a gating means uniquely
identified by said digital codes for enabling said uniquely
identified gating means to connect an incoming line to an outgoing
line.
4. The combination of claim 3 wherein each gating means is
comprised of an AND gate having two input terminals and one output
terminal, one input terminal being connected to an incoming bus bar
and the other being connected to receive a control signal from said
control signal generating means, and said output terminal being
connected to an outgoing bus bar.
5. The combination of claim 2 wherein each of said switching
matrices comprises
a common transmission channel,
a code counter for repeatedly generating a plurality of unique
digital codes in a predetermined sequence, said codes including all
codes of incoming and outgoing lines, where an incoming line is a
calling line in one switching system and the called line in the
other, and an outgoing line is the called line in the one switching
system and the calling line in the other,
a source of clock pulses for driving said counter,
a separate decoding means associated with each incoming line for
detecting means associated with each incoming line for detecting
when said counter has advanced to the code of the incoming
line,
means responsive to output signals from said decoding means for
synchronizing said converting means on said incoming lines to
operate in sequence,
means responsive to output signals from said decoding means, for
gating out in sequence digital signals from said converting means
to said common channel, thereby effecting time-division
multiplexing of digital signals from incoming lines onto said
common channel,
a separate register associated with each outgoing line adapted to
receive and store the digital code of an incoming line involved in
a telephone call,
separate means associated with each register for decoding the
digital code of an outgoing line involved in a telephone call,
separate means associated with each outgoing line for comparing the
output of said code counter with the digital code of an incoming
line stored in the associated register, and
a separate means responsive to the output of each comparator for
synchronizing separate operation of one of said reconverting means
connected to an outgoing line associated with the one of said
registers being compared, thereby demultiplexing and reconverting
to analog form digital signals time-division multiplexed onto said
common channel in digital form.
6. The combination of claim 5 wherein said converting means and
reconverting means transmit and receive respectively, digital
signals in parallel, and said common channel transmits digital
signals in parallel.
Description
BACKGROUND OF THE INVENTION
This invention relates to electronic telephone switching systems,
and more particularly to a common-control system that converts
analog signals to digital signals for the switching process, and
reconvertes the signal to analog form after processing through the
switching system.
Historically, switches have been developed to make telephone
connections in the simplest feasible way. Many switches still in
use make electro-mechanical connections. More recently, faster and
quieter electronic switches have been developed to provide
connections, but in the same basic way, which is to provide an
analog signal path through the switching matrix. Both types of
switches, electro-mechanical and electronic, have been used in
systems which employ a computer to control the switching matrix to
provide so-called computer aided switching.
Common-control electronic switching systems are coming into greater
use because of the need to minimize the size and complexity of
central office equipment. A common-control system is one in which
some equipment is time shared to complete calls by subscribers, as
opposed to direct-control systems in which dial pulses or tones
directly control originating and terminating equipments that
establish a connection in a step-by-step, progressive manner.
In a typical common-control system an off-hook signal generated by
a calling party causes a linefinder to be connected to the
terminals of the calling line. A circuit associated with the
line-finder will cause a link connector to search for an idle
register sender that is time shared in the task of completing
connections for other lines. When an idle register sender is found,
a dial tone is placed by the register sender on the calling line.
The calling party may then proceed to dial a call. The first three
dialed digits are received by the register sender and decoded to
select a trunk to the terminating office. A trunk is selected and a
connection is made with the terminating office through the trunk in
order for the next four digits dialed to be transmitted to the
terminating office to effect connection of the trunk to the called
line through a switching matrix. Some systems employ a computer to
control the switching matrix, as just noted, using either space
division multiplex or time division multiplex techniques. In either
case, the connection made through the matrix is an analog signal
connection using either electro-mechanical or solid-state
electronic switches. With an ever increasing number of subscribers,
it would be desirable to provide more economical ways of making a
large number of connections in originating and terminating
equipment free of cross talk. This invention, coupled with the
expected continuing decline in cost of digital components, will
meet this need.
SUMMARY OF THE INVENTION
In accordance with the present invention, connections are made from
line to trunk, trunk to trunk, trunk to line, or line to line, in
an electronic telephone switching system using only digital
techniques in the actual connections. For purposes of defining this
invention in the claims, a trunk is regarded to be the same as a
line. The invention depends on converting a telephone
communications signal on an incoming line from analog to digital
form, routing the digital signal to a desired outgoing line through
digital switching means, and reconverting the digital signal to
analog form before placing the switched signal on the outgoing
line. Either time-division or space-division multiplexing may be
employed in the digital switching means. In a space-division
system, the connecting means is comprised of a matrix of buffered
AND gates. One input terminal of each AND gate is connected to one
input line, and a second terminal of the AND gate is connected to a
digital select signal. Dialed numbers are decoded to provide the
digital select signal required to enable one AND gate to couple one
line to another for transmission of a communication in one
direction. A similar matrix is provided for transmission of
communications in the other direction.
In a time-division system, signals on incoming lines are
continually sampled and converted to digital form in sequence under
control of a clock pulse counter. While a give sample is being
taken from a line, the previous sample from that line is
transmitted over a channel common to all lines, each of which may
be called simultaneously. The common channel is connected to
terminating lines through digital-to-analog converters which are
activated to receive digital signals when called at times
corresponding to sampling times of the calling lines. The code of
the line called is decoded by a decoder associated with that called
line, and the output of that decoder enables the code of the
calling line to be stored in a register associated with that called
line. A comparator detects when the counter is equal to the code of
the calling line stored in the register. At that time, the sampled
and converted signal of the calling line transmitted by its
associated analog-to-digital converter over the common channel is
strobed into the digital-to-analog converter associated with the
called line. Signal traffic in the opposite direction is through a
similar, but reversed, matrix where the codes of the calling and
called lines are reversed.
The novel features that are considered characteristic of this
invention are set forth with particularity in the appended claims.
The invention will best be understood from the following
description when read in connection with the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates in a general block diagram a system through
which a telephone line is connected to a called line through
originating and terminating equipment, either or both of which
employ switching systems according to the present invention.
FIG. 2 illustrates an originating switching system (OSS) for the
system of FIG. 1 implemented with space-division multiplexing
techniques as a first embodiment.
FIG. 3 is a circuit diagram of a buffered AND gate used in the
switching systems of FIGS. 2 and 4.
FIG. 4 illustrates the manner in which hybrid junctions are used
for connecting two-wire lines and trunks to analog-to-digital and
digital-to-analog converters for two-way communications through
switching equipment.
FIG. 5 illustrates a terminating switching system (TSS) for the
system of FIG. 1 implemented in accordance with the techniques of
the first embodiment of FIG. 2.
FIG. 6 illustrates a second embodiment of an originating switching
system (OSS) using time-division multiplexing techniques.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
A typical subscriber line circuit is disclosed in FIG. 1 by way of
illustrating an environment for the invention, and not by way of
limitation. It is recognized that many different telephone systems
are in operation, each with its own design for the subscriber line
circuit, and virtually any system can benefit from the present
invention. Consequently, the subscriber line circuit is not to be
deemed a limitation on the invention. For example, some systems,
such as the Bell Systems No. 1 ESS, employ a Ferrod Line Scanner to
monitor the states of the subscriber lines. The outputs of the
Scanner are then used to determine when the line is not in use
(on-hook), when a call is initiated (off-hook), when a call is
still in progress (still off-hook) and finally when the call is
completed (on-hook again).
In this example, an off-hook condition of a telephone 10 causes
current to flow in the lines 11 connected directly to a subscriber
line circuit 12 in the central office. The subscriber line circuit
is comprised of several relays. The solenoid coil of a relay
K.sub.1 is in series with the calling lines 11 so that when the
handset is lifted, contacts in the telephone close a circuit to
energize that relay. closing the contact K.sub.1A of the relay
K.sub.1 produces a START signal to initiate a call in originating
equipment 13. Once the originating equipment begins making a
connection to a trunk 14, the solenoid of a relay K.sub.2 is
energized through closed contact K.sub.1B to deenergize the relay
K.sub.1, thus terminating the start signal. Contact K.sub.1B then
opens, but relay K.sub.2 is kept energized by closed contact
K.sub.2C.
The START signal causes a line-finder 17 to be connected to the
terminals of the calling line. A link circuit 18 associated with
the line-finder will signal a sender selector 19 to search for an
idle register sender 20 which sends back a dial tone on the calling
line 11. The caller then dials. The first three digits are stored
in a register 21 for use in a decoder-marker 22 for selection of a
local line or a trunk to a terminating office via a switching
system 23, which may be implemented in accordance with the present
invention.
While the decoder-marker 22 and switching system 23 are selecting a
trunk, the four digits of the number of the line being called are
stored in the register 21. Once a trunk to another central office
is selected and a connection is made, a line number translator 24
transmits the line number (four digits) stored in the register 21
via a selection control signaling circuit 25. For a local call to
another telephone, the decoder-marker will respond to the first
three digits dialed and select through the switching system 15 a
line designated by the next four digits dialed. In that manner, the
first three digits are used to select a trunk over which the next
four digits are sent or, in the case of a local call, to route the
next four digits to terminating equipment 15 of the originating
central office. If the central office shown in FIG. 1 is receiving
a call over a trunk 14 the terminating equipment receives the "next
four digits" for selecting a line. This can all be expanded to
accommodate direct long distance dialing in a direct and analogous
manner.
At the terminating office, equipment like the terminating equipment
15 at the originating office receives the selection control signals
as dialed numbers. To complete the general description of a call,
reference will now be made to the terminating equipment 15 in FIG.
1, assuming for that purpose that the system shown is at the
receiving office.
The selection control signals are received and stored in a register
26 for selection through a switching system 27 of the line called.
Before the selection is made, a line-busy testing circuit 28
determines whether the line called is busy. If so, it transmits a
busy signal to the originating line through the trunk selected, and
if not, the line dialed is connected through the circuit 27. The
manner in which a busy signal is generated and transmitted to the
calling line is conventional and not part of the present invention
which is directed at implementing the functions of the blocks 23
and 27 using all digital techniques.
In the event of a local call, the selection register 26 receives
the "next four digits" from the decoder marker under control of the
"first three digits." The calling lines 11 are then connected to
lines of another telephone connected to the originating and
terminating equipment in the same manner as the telephone 10.
However, a check is first made to determine if the line called is
busy as in the case of an incoming call from another central office
over a trunk.
It is evident from the foregoing that all that is required for
trunk or line switching is a register to store the trunk or line
digits for use in the digital line or trunk switching systems 23
and 27. As will become apparent the switching systems are the same
for both trunk and line connections. The difference is only that
trunk connections involve three digits for selecting one out of 999
trunks while line connections involve four digits for selecting
9999 lines (assuming all combinations except all zero's, are used
for designating a trunk or line).
Referring now to FIG. 2, an electronic switching system suitable
for the originating equipment and/or terminating equipment of a
central office will now be described. For convenience, the
line-to-trunk switching system 23 of the originating equipment will
be described because there the dialed digits are already decoded
and the result need only be combined with the number of the calling
line and the composite effectively stored in a register 30
consisting of a bank of MN flip-flops. The outputs of the
flip-flops are trunk select signals, S.sub.M,N, where the first
subscript M designates the number of the calling line and the
second subscript N designates the trunk line called. The latter is,
of course, derived by directly converting the first three dialed
digits D.sub.1, D.sub.2 and D.sub.3 through decimal-to-binary code
converter 31, and the first is derived from the number of the
calling line through an encoder 32. Because binary logic is easier
to implement, the encoder converts the assigned decimal number of a
calling line to a binary code, or more precisely generates a binary
code corresponding to the decimal line number L.sub.N, where the
subscript N is a number from 1 to 9999. A decoder 33 responds to
the binary signals M and N to generate the signals S.sub.M,N stored
in the register (bank of independent flip-flops) 30. The manner in
which that is done is straight-forward. Since the binary code M
represents the line number (L.sub.N = M), and the binary code N
represents the trunk number dialed (T.sub.N = N), the select signal
S.sub.M,N for the respective line and trunk combinations is
generated by the simple Boolean logic equation
S.sub.MN = X.sub.M .sup.. X.sub.N
where X.sub.M and X.sub.N are the logic signals,
X.sub.M = 1 if line number L.sub.N = M
0 otherwise
and
X.sub.N = 1 if trunk number T.sub.N = N
0 otherwise
For example, assume line 3 is dialing trunk 2, M is then equal to
the binary number 011 and N is then equal to the binary number 010.
A first AND gate detects the presence of M = 011 and a second AND
gate detects the presence of N = 010. A third AND gate connected to
the outputs of the first and second AND gates produces a true
signal S.sub.3,2 when the outputs of the first two AND gates are
both true.
The lines L.sub.1 - L.sub.9999 are connected by analog-to-digital
(A/D) converters to an array of horizontal conductors or bus bars
trunks T.sub.1 - T.sub.999 are connected by digital-to-analog (D/A)
converters. At each intersection of a line and trunk, there is a
buffered AND gate G having one input terminal connected to a
control signal S.sub.M,N and another input terminal connected to a
horizontal conductor coupled to the line L.sub.N by an A/D
converter. The output of the AND gate is connected to a vertical
conductor coupled to the trunk T.sub.N by a D/A converter. Such a
buffered AND gate is shown in detail in FIG. 3. Binary 1's and 0's
are represented by positive and zero voltages, respectively. Diodes
35 and 36 are normally forward biased through a resistor 37 such
that the junction between the diodes 35 and 36 is at zero volts.
Under those conditions, a diode 38 is off (insufficiently forward
biased to conduct). When the select signal S.sub.M,N is positive at
the level of the bias voltage, B+, the diode 35 is off but the
diode 36 is on (sufficiently forward biased to conduct). The
junction between the diodes 35 and 36 remains at zero volts. As the
digital output of the A/D converter transmits binary 1's, the diode
36 is turned off thereby producing binary 1's at the outputs of the
diode 38 which functions as a buffer diode.
Each line and trunk is connected to its associated A/D and D/A
converters by hybrid junctions 40 and 41, as shown in FIG. 4 for
the line L.sub.N = M and trunk T.sub.N = N. The selection matrix of
FIG. 2 is represented in FIG. 4 by the functional block OSS
(originating switching system). A functional block TSS (terminating
switching system) in FIG. 4 represents a selection matrix similar
to that of FIG. 2 and in parallel with that of FIG. 2, but for
return communication from the trunk dialed, as shown more fully in
FIG. 5. There it should be noted that the buffered AND gates at
intersections between conductors or bus bars are arranged for
transmission of digital data streams from trunks to lines. In that
manner, two-way communications is established by a digital
switching system at the originating office. The terminating office
may be equipped with other switching systems, or with the same
digital switching system as just described for an outgoing call.
All of the D/A and A/D converters of the OSS and the TSS are
synchronized in a conventional manner.
More than one line can place an outgoing call at the same time, but
not through the same trunk. Consequently, to accommodate a number k
of calls to each central office, k additional trunks having the
same three-digit number must be added to the OSS and TSS, thus
expanding the OSS and TSS to matrices of MkN gates. The code
converter 31 must then be provided with means for distributing
calls to the same central office among the k trunks.
As an example, assume calls to the central office assigned the
trunk number T.sub.N. The first call is assigned the control signal
1N, the next 2N and so forth to kN, each time assigning the lowest
free number of the series from 1 to k in the decoder 33. That may
be accomplished by a stepping switch or ring counter which steps
through stages 1 through k until it finds a free trunk. A flip-flop
is then set to indicate that trunk is busy. The output of that
flip-flop is combined with the dialed number to form the control
signal S.sub.MkN. When all trunks are busy, an all trunk busy (ATB)
signal is placed on the calling line in the conventional
manner.
When a call has been completed a disconnection or release signal is
placed on the trunk in response to an on-hook signal. That signal
is detected to reset the associated one of the MN or MkN flip-flops
used to store the select control signals S.sub.M,N or
S.sub.M,kN.
The line-to-line and trunk-to-line switching system 27 (FIG. 1) is
implemented in a manner similar to the line-to-trunk switching
system 23 insofar as handling incoming calls from another central
office is concerned. However, it is expanded to accommodate
line-to-line switching by providing additional conductors in the
OSS and TSS matrices parallel to the conductors for the trunk
lines, one additional conductor for each line of the local central
office. The selection register 26 comprises an expanded MN or M,kN
register and associated logic which is similar to that described
with reference to FIG. 2 for the line-to-trunk switching system,
except that now the incoming trunks and lines are encoded (M) while
dialed telephone number (last four digits) are converted from
decimal to a binary code (N) to provide the control signals
S.sub.M,N or S.sub.M,kN, where k is now a multiplier for M, or at
least to that part of M relating to incoming trunks. Local incoming
lines are limited to the number of lines that can be accomodated by
the four dialed digits.
A second embodiment using time-division multiplexing techniques
will now be described. For convenience, an originating switching
system (OSS) corresponding to that of FIG. 2 will be used as the
example. It employs a common channel (or bus) 60 as shown in FIG. 6
for transmission from any A/D converter, such as a converter 61, to
any D/A converter, such as converter 62.
A binary counter 63 counts clock pulses from a source 54 to cycle a
binary output code C.sub.1 through C.sub.n, where 2.sup.n is equal
or greater than the number M of analog-to-digital converters. That
n-bit code is decoded by suitable decoders assigned to the A/D
converters, such as a decoder 65 assigned to the A/D converter 61.
In this manner the output of the A/D converters is sampled once
during each cycle of the counter.
The conversion process requires a sequence of steps which occurs in
two parts. The first part is to sample the analog signal and to
store the sample in a capacitor. A typical sample-and-hold circuit
may be used in which the storage capacitor is first discharged and
then charged to a level proportional to the analog input. The
charge is then held until the next sampling period.
While the charge is held, the second part takes place, which is to
convert the sample to digital form. A well known successive
approximation process of conversion is ideally suited since the
period for conversion always requires the same number of steps for
any level of analog input.
All steps of both parts may be carried out during one period of the
clock pulse source by gating synchronized clock pulses from a
higher frequency source into sequence control logic of the A/D
converter during the period it is enabled by its associated
decoder. However, several periods of the clock pulse source may be
used to complete both parts of the conversion process, so long as
the process is complete before the counter 63 recycles.
It is significant to note that during the first part of the
conversion process, the last sample has already been converted and
is stored in digital form in a register used in the actual
analog-to-digital conversion process. Consequently, a convenient
time to enter new digital signals into the D/A is during the
sampling part of the conversion process in the respective A/D's
because the digital outputs from each is stable then.
During the sampling period, an AND gate is enabled, such as an AND
gate 66, to transmit the output of the converter to the common
channel. If the transmission is serial through a single gate, the
bus bit rate capacity must be at least j MR, where 2.sup.j is the
number of quantizing levels (typically 64), M the number of A/D
converters to be multiplexed and R is the sampling rate (typically
8,000 samples per second) for good voice transmission. However, in
the case of parallel transmission as is assumed here, the required
capacity of each separate parallel bit path would be only MR.
Parallel transmission is assumed for purposes of description, and
for simplicity, a single gate 66 represents a bank of j gates, and
the channel 60 includes j parallel bit paths.
The called trunk number D.sub.1 D.sub.2 D.sub.3 is converted to a
code N which is decoded by a decoder associated with the trunk
T.sub.N, such as a decoder 67 associated with the trunk connected
to the D/A converter 62. That enables a register, such as a
register 68, to enter in parallel the code M of the calling line.
Thereafter, each time the counter 63 reaches the count
corresponding to the code of the calling line, a comparator 69
enables an AND gate 70. The next clock pulse is transmitted through
the AND gate to cause the D/A converter 62 to enter the digital
signals transmitted from the A/D converter of the calling line. In
the D/A converter, a buffer register accepts and stores the digital
signals. A suitable ladder network connected to the output
terminals of the buffer register then provides static
digital-to-analog conversion between sample pulses, i.e., between
periods for entry of new digital signals. When a call is complete
the register 68 is cleared by the "on-hook again" signal.
A terminating switching system (TSS) would be implemented in the
same way but reversed (considering the called trunk the line and
the calling line the trunk) such that the code M is decoded to
store the code N for comparison, and operating the A/D converters
connected to the trunks in the same manner as the A/D converters
connected to the lines. The TSS will, of course, have a separate
"common channel."
It is noted that utilization of time-division multiplexing also
provides a simple method for allowing several users to participate
in a conference call. Each conferee, instead of receiving only one
time slot from the common bus, is fed by several time slots
(corresponding to the sampling time for the other conferees). This
may be accomplished by means of a "Conference" comparator connected
by the operator or auxiliary switching means to the D/A converter
of each conferee. Control signals from the operator set this
comparator to activate the selected D/A converter and accept the
digital signals from all of the conferees. It is noted further that
in most cases all subscribers would not use the switching system
simultaneously and therefore an arrangement of line finders or
concentrators would be employed in conjunction with the switching
system described herein. This approach is normal in current
telephone practice and for this invention would greatly reduce the
total number of D/A and A/D converters and hybrid junctions
required.
While the present invention has been described with reference to
particular special purpose computer embodiments, it should be
understood that practice of the invention is not limited to those
embodiments. For example, the time-division multiplexing system
described with reference to FIG. 6 may be implemented with
software, using a programmable digital computer, rather than with
hardwired components in the configuration shown. Consequently, it
is not intended that the scope of the invention be determined by
the embodiments disclosed, but rather should be determined by the
breadth of the appended claims.
* * * * *